1. Field of the Invention
The present invention relates to an optical pickup for optically recording and reproducing information on and from a record medium, such as an optical disk or the like, and more particularly relates to an optical pickup having a plurality of light sources for emitting a plurality of kinds of light beams whose wave lengths are different from each other, and further relates to an information reproducing apparatus and an information recording apparatus containing this optical pickup.
2. Description of the Related Art
Recently, an optical disk has been developed which can have the same diameter as those of CD and DVD and can further record information at a higher density than that of the DVD (hereafter, this is referred to as xe2x80x9ca high dense optical diskxe2x80x9d. The DVD is an optical pickup having a record capacity equal to about seven times that of the CD. However, a memory capacity of the high dense optical disk exceeds that of the DVD. In this high dense optical disk, for example, about 15 gigabytes of information can be recorded on only one surface.
In parallel with the development of the high dense optical disk, it is considered to use a blue laser diode as a light source of a light beam to record and reproduce information. An oscillation wave length of this blue laser diode is 430 nm. It is shorter than that of a red laser diode (having an oscillation wave length of 650 nm) which is presently used for the record and reproduction of the DVD and the CD. As for the blue laser diode, for example, a blue semiconductor laser for using InGaN as an activation layer and an SHG laser for using LiNbO3 as an SHG (Second Harmonic Generation) layer have been put into practical use.
It is desirable to use a blue laser light beam emitted from the blue laser diode, when recording the information on the high dense optical disk and when reproducing the information from the high dense optical disk.
The high dense optical disk is an optical pickup having the appearance similar to those of the CD and the DVD. Thus, it is desirable to record the information on the high dense optical disk and reproduce the information from the high dense optical disk by using a DVD player, a CD player or a CD/DVD compatible player.
On the other hand, an optical disk having an information record surface composed of two or more layers is developed in order to further improve the record capacity of the optical disk. Accordingly, the memory capacity of the optical disk can be increased by two times or more. The DVD having such two-layer structure is being realized.
An intermediate layer for dividing the two-layer information record surface is required to form the two-layer information record surface on the optical disk. Thin film made of gold (Au) is typically used as the intermediate layer. This reason is that in a case of the gold intermediate layer, since a degree of absorbing the red laser light beam is small, a stable laser light beam can be obtained.
However, the gold intermediate layer has the high degree of absorbing the blue laser light beam. Thus, the employment of the blue laser light beam disables the utilization of the gold intermediate layer. Actually, it is known that the employment of a laser light beam having a wave length of about 450 nm or less disables the utilization of the gold intermediate layer. As mentioned above, the blue laser light beam is required in order to record the information on the high dense optical disk or reproduce the information from the high dense optical disk. Hence, in the case of the high dense optical disk having the two-layer structure, it is necessary to use an intermediate layer other than the gold intermediate layer.
Thus, for example, in order to achieve a compatible type memory reproducing apparatus which can record and reproduce on and from both optical disks of the high density optical disk having the two-layer structure and the DVD having the two-layer structure, it is necessary to mount a blue laser light source and a red laser light source within an optical pickup and then switch between them based on the type of optical disk.
Next, an objective lens is required in order to collect the laser light beam onto the optical disk. In order to realize a small optical pickup having the blue laser light source and the red laser light source, it is desirable to collect the blue laser light beam and the red laser light beam through a single objective lens. However, if the two laser light beams are collected through the single objective lens, the dispersion resulting from wave length dependence of a refractive index occurs in each laser light beam. The occurrence of the dispersion causes chromatic aberration in each laser light beam. As a result, there may be a case that the laser light beam can not be excellently collected.
FIG. 1 is a graph showing the relationship between a wave front aberration RMS (a vertical axis) and an angle of an image height (a horizontal axis) in a blue laser light beam and the relationship between a wave front aberration RMS and an angle of an image height in a red laser light beam. Data on the graph is obtained in such a way that optical material typically used because of traditionally small dispersion is used to create an objective lens and then this objective lens is used to collect the red laser light beam and the blue laser light beam in an optimal condition. In addition, the angle of the image height implies an angle between a direction of a perpendicular of the objective lens and an optical axis of an incident laser light beam. The wave front aberration RMS implies a root mean square of an error between an ideal wave front without aberration and an actual wave front.
As shown in FIG. 1, when the image height is 0 degree, the chromatic aberration can be sufficiently compensated for both the laser light beams. However, as the image height becomes higher, each chromatic aberration is deteriorated. Actually, in the case of the red laser light beam, when the image height exceeds 0.6 degrees, the wave front aberration RMS exceeds 0.07. This value of 0.07 is a value known as a so-called Marshall limitation. When the wave front aberration exceeds 0.07, the function as the optical pickup can not be carried out any longer.
In addition, when gathering the data on the graph of FIG. 1, the blue laser light beam is made to enter the objective lens as collimated light (infinite system (afocal system)), in order to suppress as much as possible the chromatic aberration in each laser light beam. On the other hand, the red laser light beam is made to enter the objective lens as diffusion light or convergence light (finite system (focal system)).
Then, it may be considered to create the objective lens by using material having the smaller dispersion than those of conventional optical materials, for example, fused quartz or fluorite. However, in this case, a material cost is expensive, and a fused temperature is high, which causes the molding for the objective lens to be difficult, and thereby reduces the manufacturing yield.
Moreover, the following two techniques may be considered as the technique of reducing the manufacturing cost, making the molding process easier and ensuring the compatibility between the high dense optical disk and the DVD. The first technique is a technique of using a hologram element, making optical paths of the respective laser light beams different from each other (refracting them so that they are not parallel to each other) and then collecting the respective laser light beams. The second technique is a technique of providing a notch or a groove in a part of a transmission surface of an incident side of the objective lens.
However, although the first technique can perfectly remove the chromatic aberration and also create the optical pickup at a small size and a low cost, it has the problem that the utilization of the hologram causes a loss of a light quantity in each laser light beam to be large. The first technique further has the problem that it is difficult to remove a laser light beam other than the normal laser light beam incident on a detector (This is a laser light beam resulting from the fact that the original light beam is refracted by the hologram. This may be typically referred to as a stray light).
According to the second technique, although the optical pickup can be created at a small size and a low cost similarly to the first method, the loss of the light quantity in each laser light beam is still large. Also, the second technique has the problem that if the optical disk is tilted with respect to the optical axis of the objective lens, the light collection performance is extremely deteriorated.
It is therefore an object of the present invention to provide an optical pickup which can collect a plurality of kinds of light beams without substantial aberration, compensate the chromatic aberration with a small size and a simple structure and compensate the chromatic aberration without inflicting a loss of a light quantity on each light beam, and an information reproducing apparatus and an information recording apparatus which contain the pickup.
An optical pickup in accordance with the present invention includes: a light source for emitting a plurality of light beams whose wavelengths are different from each other; and an objective lens comprising a first lens for collecting each of the plurality of light beams onto a recording medium, and a second lens for compensating chromatic aberration in the plurality of light beams. The second lens is placed on the first lens in contact and is located across an optical path of each of the plurality of light beams.
Although the objective lens has the first lens and the second lens, it is a single unit because the second lens is placed on the first lens in contact. The objective lens in accordance with the present invention has at least two functions. Namely, it functions as a lens for collecting a plurality of light beams whose wavelengths are different from each other onto the recording medium. Also, it functions as a device for compensating chromatic aberration in a plurality of light beams whose wavelengths are different from each other. By using such an objective lens in the optical pickup, it is possible to create the optical pickup that does not inflict substantial aberration on each light beam.
For example, it is assumed that the optical pickup in accordance with the present invention is used in an information reproducing apparatus which is designed to reproduce information recorded on a plurality of kinds of recording medium by using a plurality of light beams whose wavelengths are different from each other. In this case, by using the objective lens in accordance with the present invention, each of the light beams can be collected on the recording medium accurately, and chromatic aberration in the light beams can be compensated. Therefore, optimum focusing on the recording medium can be achieved with respect to each light beam. Consequently, the accuracy of the reproduction of the information with respect to each recording medium can be increased.
Furthermore, the objective lens in accordance with the present invention is a single unit including the first lens and the second lens. Therefore, it is possible to create the optical pickup that can compensate the chromatic aberration without largely changing its structure, and also possible to miniaturize the optical pickup itself.
Moreover, the objective lens in accordance with the present invention can compensate chromatic aberration without inflicting the loss of the light quality on each light beam, because it does not use a hologram or a lens having a transmission surface on which notch or a groove is formed.
In the optical pickup according to the present invention, the second lens may be made of ultraviolet curing resin. Therefore, the second lens can be created by coating the ultraviolet curing resin and then curing it. Hence, it is possible to create the second lens having the accurate form in a simple process.
In the optical pickup according to the present invention, an absolute value of a difference between an Abbe number of the first lens and an Abbe number of the second lens is greater than 10, and each of the Abbe numbers is defined by using a refractive index with respect to helium d line, a refractive index with respect to mercury F line and a refractive index with respect to mercury C line. Therefore, it is possible to compensate the chromatic aberration occurring in the light beams.
In the optical pickup according to the present invention, the second lens has a first surface and a second surface which are opposite each other and both of which are located across the optical path of each of the plurality of the light beams, the first surface is in contact with the first lens, and the second surface is flat. Therefore, it is possible to prevent eccentricity from occurring between the first lens and the second lens, when the second lens is placed on the first lens to then create the objective lens. Hence, the objective lens which can surely compensate the chromatic aberration can be created in a simple process.
In the optical pickup according to the present invention, an central axis of the first lens corresponds to an central axis of the second lens, the second lens has a first surface and a second surface which are opposite each other and both of which are located across the optical path of each of the plurality of the light beams, the first lens has a third surface and a fourth surface which are opposite each other and both of which are located across the optical path of each of the plurality of the light beams, each of the first surface, the second surface, the third surface and the fourth surface is an axisymmetric aspheric-surface whose symmetry axis corresponds to each of the central axis of the first lens and the central axis of the second lens. Therefore, each of the light beams can be effectively collected while miniaturizing the objective lens.
In the optical pickup according to the present invention, an central axis of the first lens corresponds to an central axis of the second lens, the second lens has a first surface and a second surface which are opposite each other and both of which are located across the optical path of each of the plurality of the light beams, the first lens has a third surface and a fourth surface which are opposite each other and both of which are located across the optical path of each of the plurality of the light beams, each of the first surface, the second surface, the third surface and the fourth surface is a spherical surface. Therefore, each of the light beams can be effectively collected while miniaturizing the objective lens.
The nature, utility, and further feature of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.